Xylene isomers are important intermediates in chemical syntheses, and specific xylene isomers are desired for different processes. Paraxylene is a feedstock for terephthalic acid, and terephthalic acid is used in the manufacture of synthetic fibers and resins. Metaxylene is used in the manufacture of certain plasticizers, azo dyes, and wood preservatives. Orthoxylene is a feedstock for phthalic anhydride production, and phthalic anhydride is used in the manufacture of certain plasticizers, dyes, and pharmaceutical products.
Xylene isomers typically are separated from mixed xylene streams by using an adsorbent selective to the desired isomer. The desired isomer is adsorbed, and the remaining isomers are discharged in a mixed raffinate stream. A desorbent is then used to desorb the desired xylene isomer, and the desorbent and desired xylene isomer are collected and separated by distillation (also referred to as fractionation). The desorbents are typically referred to as either heavy or light, where a heavy desorbent has a higher molecular weight and a higher boiling point than xylene and a light desorbent has a lower molecular weight and a lower boiling point than xylene. Xylene isomer recovery systems with heavy desorbents tend to use less energy than systems with light desorbents because the desorbent does not need to be repeatedly evaporated and lifted in the fractionation step. However, heavy desorbent systems require stringent feed purity to control the accumulation of undesired compounds in the recycled desorbent. The undesired compounds are impurities that reduce the desorbent effectiveness and product purity. Additional equipment is needed to maintain the heavy desorbent purity during the desorbent recycling process. The distillation columns in heavy desorbent systems have higher reboiler temperatures, which leads to higher operating pressures. These higher operating pressures require higher pressure ratings for the equipment involved, which increase the equipment capital costs and maintenance expenses.
A light desorbent system allows a relaxed feed specification relative to a heavy desorbent system. This helps to offset the increased energy costs associated with recovering light desorbent as a distillation column overhead stream. The light desorbent systems also provide substantial savings in the total equipment count for xylene recovery systems, because the additional equipment for desorbent storage and recovery is not needed. The light desorbent xylene recovery systems also have lower distillation column operating pressures, so thinner shell thicknesses and lower pressure ratings can be used to further reduce capital costs for installing new systems. Toluene is one example of a light desorbent that can be used, and toluene is less expensive than many of the heavy desorbents available.
As such, in the prior art, a trade-off exists between systems that use heavy desorbents and systems that use light desorbents. In situations where the feed specification is not able to be tightly controlled, it may be desirable to use a light desorbent system as opposed to a heavy desorbent system for the reasons discussed above. However, the prior art fails to disclose any systems that address the undesirable increase in energy use associated with such light desorbent systems.
Accordingly, it is desirable to develop methods and systems for producing selected xylene isomers from mixed xylene streams using light desorbents in a manner that reduces overall energy consumption. Furthermore, other desirable features and characteristics of the present embodiment will become apparent from the subsequent detailed description and the appended claims, taken in conjunction with the accompanying drawings and this background.